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<title>Chaining up: GObject Reference Manual</title>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-gobject-chainup"></a>Chaining up</h2></div></div></div>
<p>Chaining up is often loosely defined by the following set of
    conditions:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Parent class A defines a public virtual method named <code class="function">foo</code> and 
        provides a default implementation.</p></li>
<li class="listitem"><p>Child class B re-implements method <code class="function">foo</code>.</p></li>
<li class="listitem"><p>B’s implementation of <code class="function">foo</code> calls (‘chains up to’) its parent class A’s implementation of <code class="function">foo</code>.</p></li>
</ul></div>
<p>
      There are various uses of this idiom:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>You need to extend the behaviour of a class without modifying its code. You create
          a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
          and chain up to ensure that the previous behaviour is not really modified, just extended.
          </p></li>
<li class="listitem"><p>You need to implement the
          <a class="ulink" href="http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern" target="_top">Chain
          Of Responsibility pattern</a>: each object of the inheritance
          tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that
          each handler is run in turn.</p></li>
</ul></div>
<p>
    </p>
<p>
      To explicitly chain up to the implementation of the virtual method in the parent class, 
      you first need a handle to the original parent class structure. This pointer can then be used to 
      access the original virtual function pointer and invoke it directly.
      <a href="#ftn.id-1.6.3.9.3.1" class="footnote" name="id-1.6.3.9.3.1"><sup class="footnote">[7]</sup></a>
    </p>
<p>
      Use the <code class="function">parent_class</code> pointer created and initialized
      by the
      <a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE:CAPS" title="G_DEFINE_TYPE()"><code class="function">G_DEFINE_TYPE</code></a>
      family of macros, for instance:
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
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14</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="gtkdoc kwb">static void</span>
<span class="function">b_method_to_call</span> <span class="gtkdoc opt">(</span>B <span class="gtkdoc opt">*</span>obj<span class="gtkdoc opt">,</span> gint some_param<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
  <span class="comment">/* do stuff before chain up */</span>

  <span class="comment">/* call the method_to_call() virtual function on the</span>
<span class="comment">   * parent of BClass, AClass.</span>
<span class="comment">   *</span>
<span class="comment">   * remember the explicit cast to AClass*</span>
<span class="comment">   */</span>
  <span class="function">A_CLASS</span> <span class="gtkdoc opt">(</span>b_parent_class<span class="gtkdoc opt">)-&gt;</span><span class="function">method_to_call</span> <span class="gtkdoc opt">(</span>obj<span class="gtkdoc opt">,</span> some_param<span class="gtkdoc opt">);</span>

  <span class="comment">/* do stuff after chain up */</span>
<span class="gtkdoc opt">}</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
  </p>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id-1.6.3.9.3.1" class="footnote"><p><a href="#id-1.6.3.9.3.1" class="para"><sup class="para">[7] </sup></a>
          The <span class="emphasis"><em>original</em></span> adjective used in this sentence is not innocuous. To fully 
          understand its meaning, recall how class structures are initialized: for each object type,
          the class structure associated with this object is created by first copying the class structure of its
          parent type (a simple <code class="function">memcpy</code>) and then by invoking the <code class="function">class_init</code> callback on
          the resulting class structure. Since the <code class="function">class_init</code> callback is responsible for overwriting the class structure
          with the user re-implementations of the class methods, the modified copy of the parent class
          structure stored in the derived instance cannot be used. A copy of the class structure of an instance of the parent
          class is needed.
        </p></div>
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